Shutterless injection molding method and apparatus

ABSTRACT

There is provided a method of injection molding a metal part and an injection molding apparatus which reduces or eliminates drooling of metal from a nozzle. The method includes the steps of (A) separating an injection nozzle of an injection chamber from contacting a mold surface, (B) retracting a plunger in the injection chamber to create a suction in the injection chamber, (C) closing an inlet to the injection chamber to seal the injection chamber, (D) maintaining melted metal in the injection chamber with a pressure difference and surface tension without substantial drooling from the injection nozzle, (E) placing the injection nozzle in contact with the mold surface and (F) advancing the plunger in the injection chamber to inject the metal into the mold.

FIELD OF THE INVENTION

[0001] The invention generally relates to a method and apparatus for manufacturing metallic parts, and more particularly to a method and apparatus for manufacturing metallic parts by a process involving injection of a melted metal into a mold.

BACKGROUND OF THE INVENTION

[0002] One conventional method used to produce molded metallic parts from melted metal is by die casting. A typical die casting machine and method is described in U.S. Pat. No. 5,983,976, hereby incorporated by reference. Die casting methods inject liquid metal into a mold.

[0003] Semi-solid methods for producing molded metallic parts differ from the die casting methods by injection molding a metal in its semi-solid state rather than in its liquid state. Semi-solid methods are disclosed in U.S. Pat. Nos. 3,902,544 and 3,936,298, both of which are incorporated by reference herein.

[0004] Both liquid die casting and semi-solid injection molding methods require that the metal to be sufficiently fluid in order to flow into the mold. Further, in conventional injection molding machines (FIGS. 1a, 1 b) the injection chamber is oriented horizontally. The result of this combination of features is that conventional injection molding machines tend to drool melted metal out of the injection nozzle between injection steps.

[0005] In response to this problem, several techniques have been developed to reduce drooling. In the prior art apparatus illustrated in FIG. 1 a, metal is prevented from drooling out of the injection nozzle 90 between injection steps by forming a metal plug 91 in the exit opening 92 of the nozzle 90. However, the plug 91 is undesirable because it is injected into the sprue cavity in the mold, thus blocking the metal from flowing into the mold cavity. This tends to adversely affect the filling of the mold cavity. For example, the plug 91 or its fragments entangled in the sprue cavity may hinder the flow of molten metal, and the molten metal is disturbed when injected into the mold cavity.

[0006] To overcome the problems associated with the formation of a plug 91, a shutter mechanism is developed. A typical shutter mechanism 95 is illustrated in FIG. 1b. Metal is prevented from drooling out of the injection nozzle between injection cycles by closing the shutter 95 between the exit opening 92 of the nozzle 90 and the mold 94. Although the shutter 95 reduces metal from drooling out of the exit opening 92, its use increases injection cycle time. The use of the shutter results in a relatively long injection cycle time. Further, unless the shutter 95 is exactly flush with the tip of nozzle 90, some metal may drool out of opening 92.

[0007] Therefore, an improved system for injection molding which does not require plug formation is desirable. Preferably, the system is also capable of operating without the shutter. In addition it is preferable that the system is simple, fast and reliable.

SUMMARY OF THE INVENTION

[0008] A preferred aspect of the present invention provides a method of injection molding a metal part comprising: (A) separating an injection nozzle of an injection chamber from contacting a mold surface, (B) retracting a plunger in the injection chamber to create a suction in the injection chamber, (C) closing an inlet to the injection chamber to seal the injection chamber, (D) maintaining melted metal in the injection chamber with a pressure difference and surface tension without substantial drooling from the injection nozzle, (E) placing the injection nozzle in contact with the mold surface and (F) advancing the plunger in the injection chamber to inject the metal into the mold.

[0009] Another preferred aspect of the present invention also includes a method of injecting melted metal into a mold comprising: introducing the melted metal into a barrel, allowing at least a first portion of the melted metal to pass through said barrel into an injection chamber and injecting the melted metal from the injection chamber into the mold, wherein during the step of injecting, a pressure in the injection chamber does not decrease.

[0010] Another preferred aspect of the present invention also includes an injection molding apparatus comprising an injection chamber, a plunger in the injection chamber, an injection nozzle in fluid communication with the injection chamber having an opening sufficiently small to substantially prevent melted metal from drooling from the injection nozzle by a pressure difference between outside atmosphere and the injection chamber and surface tension.

[0011] Another preferred aspect of the present invention includes an injection molding apparatus, comprising an injection chamber containing an injection nozzle, a first means for separating the injection nozzle from contacting a mold surface and for placing the injection nozzle in contact with the mold surface, a second means for injecting a melted metal from the injection chamber into the mold and for creating a suction in the injection chamber such that the melted metal is maintained in the injection chamber without substantially drooling from the injection nozzle between injection steps due to a pressure difference between an outside atmosphere and the injection chamber, a third opening means in the injection nozzle for maintaining the melted metal in the injection chamber without substantially drooling from the injection nozzle between injection steps due to surface tension, and a fourth means for closing an inlet to the injection chamber to seal the injection chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The foregoing and other features, aspects and advantages of the present invention will become apparent from the following description, appended claims and the exemplary embodiments shown in the drawings, which are briefly described below. It should be noted that unless otherwise specified like elements have the same reference numbers.

[0013]FIG. 1a is a side view of a first prior art apparatus.

[0014]FIG. 1b is a side view of a second prior art apparatus.

[0015]FIG. 2 is a schematic illustration of a side view of an injection molding system according to a preferred embodiment of the invention.

[0016]FIG. 3 is a schematic illustration of a side view of an injection nozzle according to a preferred embodiment of the invention.

[0017]FIG. 4a is a plot of the pressure profile during the injection step of an embodiment of the invention.

[0018]FIG. 4b is a plot of the pressure profile during the injection step according to a prior art method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] In the discussion of the preferred embodiments which follows, a metal part is produced by injection molding a magnesium (Mg) alloy in a liquid state. The invention is not limited to processing of Mg and is equally applicable to other types of materials, metals and metal alloys, in a liquid or semi-solid state. A wide range of such metals and alloys are potentially useful in this invention, including magnesium (Mg), Mg alloys, aluminum (Al), Al alloys, zinc (Zn), Zn alloys, and the like.

[0020] The terms “melted metal” and “melted material” as used herein encompasses metals, metal alloys and other materials in a liquid or semi-solid state which can be processed in an injection molding system. The term “without substantial drooling” means completely without drooling or with only minimal drooling, such as only with a few drops of melted metal (rather than a steady stream of melted metal) drooling from the injection nozzle between injection shots.

[0021] Specific temperatures and temperature ranges cited in the following description of the preferred embodiments are applicable to the preferred embodiment for processing a Mg alloy in a liquid state, but could readily be modified in accordance with the principles of the invention by those skilled in the art in order to accommodate other metals and metal alloys in liquid or semi-solid state. For example, some Zn alloys become liquid at temperatures above 450° C., and the temperatures in the injection molding system can be adjusted for processing of Zn alloys.

[0022] The present inventor has determined that metal drooling out of the tip of an injection nozzle can be reduced or eliminated without using a plug or a shutter. More specifically, by creating and maintaining suction in the injection chamber between injection steps, forces are created which tend to hold the metal in the injection chamber. Further, by providing a nozzle with an appropriately sized opening (i.e., aperture or hole), the various forces acting on the metal, i.e. suction, air pressure and surface tension, may be balanced to reduce or prevent drooling. Thus, drooling can be reduced or eliminated without resorting to use of plugs or shutter mechanisms.

[0023]FIG. 2 is a schematic illustration of a side view of an injection molding system 10 according to a preferred embodiment of the invention. The system 10 includes an injection molding apparatus 12 and a mold 13. The apparatus 12 is mounted on wheels and or rails (not shown) such that it may be retracted from the mold 13 after each injection step and advanced toward the mold 13 before each injection step by a motor or hydraulics (not shown). A feeder 23 is provided with at least one heating element 25 disposed around its outer periphery. The heating element 25 may be of any conventional type. The heating element 25 operates to maintain the feeder 23 at a temperature high enough to keep the metal alloy supplied through the feeder 23 in a liquid state. For a Mg AZ91 alloy, this temperature would be about 610° C. or greater. Preferably, sufficient metal should be kept in the feeder 23 to supply about 20 times the volume needed for one injection cycle (or shot). This is because the amount of time required to melt the metal necessary for one injection cycle is longer than the injection cycle time, which depends on the volume of the mold cavity, diameter of the injection chamber and actions of the machine operator.

[0024] In a preferred aspect of the invention, the feeder 23 further contains an outlet screening element 24. For example, as illustrated in FIG. 2, the screening element 24 may comprise at least one non-horizontal wall 26, a top cover or portion 28 and an outlet port 29. Preferably, the outlet port 29 is located in one of the walls 26 instead of in the top 28 of the screening element 24. The screening element 24 may contain one wall 26 if the element 24 has a cylindrical shape, or plural walls 26 if the element 24 has a polygonal shape. Furthermore, the non-horizontal wall 26 is preferably exactly vertical or substantially vertical (i.e., deviating by about 1-20 degrees from vertical). The screening element 24 prevents solid metal pieces or ingots as well as other residue present in the melted metal from clogging the outlet port 29 because the outlet port 29 is raised from the bottom of the feeder 23. However, the screening element 24 may be omitted, if desired.

[0025] The melted metal is subsequently supplied into a temperature-controlled barrel 30 by way of gravity through a feeder port 27 which may optionally be supplied with a valve serving as a stopper (not shown). Preferably, no valve is present. A ram 32 is arranged coaxially with the barrel 30 and extends along the center axis of the barrel 30. The outer diameter of the ram 32 is smaller than the inner diameter of the barrel 30 such that melted metal flows in the space between the ram 32 and the barrel 30. The ram 32 is controlled by motor 33 for axial movement in both retracting and advancing directions along the barrel 30 and for rotation around its own axis if stirring of the melted metal is desired inside barrel 30.

[0026] In the preferred embodiment of the invention, the ram 32 includes supporting ribs or fins 34. The fins 34 are preferably attached to the ram 32 and can slide on the inner circumference of the barrel 30, both coaxially with the length of the barrel and or in a circular motion about the barrel axis. The movement produces a rotation of the fins 34 around the inner circumference of the barrel 30. Alternatively, the fins 34 may be attached to the inner circumference of the barrel 30 in such a manner as to allow the bare ram 32 to slide by. The fins 34 can be made of the same material as the ram 32 or from a different material that can withstand the required process temperatures. The fins prevent the ram 32 from tilting and wobbling away from the barrel axis. They also second enhance the uniform temperature distribution of the melted metal.

[0027] The ram 32 as shown in FIG. 2 has a pointed tip, but any shape may be used, including a blunt end or a rounded end. Preferably, the tip of ram 32 has a shape capable of blocking inlet port 37 to prevent the flow of melted metal between barrel 30 and injection chamber 50, when the ram 32 is fully advanced inside barrel 30. The injection chamber 50 contains a plunger or piston 45 and an injection nozzle 57. The plunger 45 is advanced in the injection chamber 50 by a motor or hydraulics (not shown) to inject the liquid or semi-solid metal from the injection chamber 50 through the nozzle 57 into a mold cavity 15 in mold 13. The plunger 45 contains a seal, such as O-ring(s) 41, to form an air tight seal with the inner surface of the injection chamber 50. This allows the plunger 45 to create a suction in the injection chamber 50 when the plunger 45 retracts.

[0028] An injection molding method using system 10 will now be described. After injection (i.e. after a shot), the nozzle 57 is separated from the mold 13. Preferably, this is accomplished by moving the injection molding apparatus 12 away from a stationary mold 13 die. After or during the time the injection molding apparatus 12 is retracted, the ram 32 is retracted in the barrel 30 (but may continue rotating if rotation is being used to stir the melted metal inside barrel 30). The plunger 45, which is housed in the injection chamber 50, begins retracting (moved to the right as shown in FIG. 2) to expand the volume of the injection chamber 50 to a desired volume according to the dimensions of the desired molded part before or while the apparatus 12 is retracted. The plunger 45 retraction is stopped when the volume of the injection chamber 50 becomes equal to the desired injection volume. The plunger 45 may be retracted while that ram 32 is being retracted or after ram 32 has been retracted to a desired position. While being retracted, the plunger 45 acts like a pharmaceutical syringe that draws in liquid from a container of liquid. Specifically, as the plunger 45 retracts, it creates a suction to draw in melted metal from the barrel 30 into the injection chamber 50 through port 37. The suction prevents or reduces the drooling from nozzle 57.

[0029] After plunger 45 retraction is stopped, the ram 32 is advanced downward. As a result, any metal collected in a lower portion of barrel 30 is pushed into the injection chamber 50 through the inlet port 37. The ram 32 preferably advances through barrel 30 until its end closes off inlet port 37. The ram 32 preferably remains in this position to keep inlet port 37 sealed off until injection is complete and the next shot cycle is started. The advanced ram 32 prevents metal and gases from flowing between barrel 32 and chamber 50.

[0030] In the preferred embodiment of the invention, some suction remains in the injection chamber 50 after inlet port 37 is sealed by the ram 32. Thus, the whole back side of the injection chamber 50 is sealed off. The ram 32 seals off the inlet port 37, while the seal 41 on plunger 45 seals off the back of the injection chamber 50. Thus, because the backside of chamber 50 is sealed off, a pressure difference is created between the outside air pressure on the front of the metal located in the nozzle 57 and the back of the metal in the injection chamber 50. The pressure difference acts to maintain the liquid or semi-solid metal in the nozzle 57. In addition, the inventor has recognized that there is a capillary force in the nozzle 57 acting on the metal due to the surface tension. This also tends to maintain the liquid metal in the nozzle 57. Thus, the inventor has determined that by providing the injection nozzle 57 with a sufficiently small exit opening or aperture 58, the pressure difference between the outside air pressure and the suction pressure combined with the surface tension of the melted metal can reduce or prevent the metal drooling out of the opening 58.

[0031] To inject the metal into the mold 13, the plunger 45 is advanced in chamber 50, to force the metal in chamber 50 through the nozzle 57 and the sprue cavity 14 into the mold cavity 15. After a pre-set dwell time, the two mold 13 die are separated (i.e., opened) and the molded metallic part is removed, so that a new cycle can begin.

[0032] Initially, when the tip of the nozzle 57 separates from the mold 13, parts of the solidified sprue may extend into the nozzle 57. However, because the tip of the nozzle 57 is heated above the liquidus temperature of the metal, any solid metal in the nozzle 57 is quickly melted while the apparatus 12 is being retracted, such that no plug forms. The solidified sprue, if present, may assist in reducing or preventing drool between the injection step and plunger retraction step. However, the solidified sprue is not the same as the plug 91 in FIG. 1a, because any portion of the sprue is remelted as the nozzle 57 is retracted from the mold. In contrast, the plug 91 is maintained in the nozzle 57 during the entire period between injection steps. The plug 91 is injected into the sprue cavity rather than being remelted.

[0033]FIG. 3 illustrates a side view of an injection nozzle 57 according to a preferred embodiment of the invention. The injection nozzle 57 is provided with an exit opening 58 of a predetermined size selected based on the amount of suction that will be used for the injection molding of a particular metal part. The amount suction is generally determined by the size of the part, i.e. the amount of metal required. By knowing the weight of the metal to be injected, the surface tension of the metal, the ambient air pressure, and the suction force, a desired exit opening 58 size can be determined. Thus, the size of the opening 58 is sufficiently small to substantially prevent melted metal from drooling from the injection nozzle 57 by a pressure difference between outside atmosphere and the injection chamber 50 and by surface tension. The maximum opening size which reduces or prevents drooling is preferably 15 mm or less, most preferably 10-13 mm. However, other opening sizes may be used depending on the processing conditions.

[0034] As shown in FIG. 2, heating elements 25 and 70 a-70 j are provided along the lengths of the feeder 23, the barrel 30 and the injection chamber 50. The temperature in the feeder differs depending on the material present in the feeder. For the AZ91 Mg alloy, heating elements 25 are preferably controlled so that the temperature in the feeder 23 is about 640 to 670° C. near the upper surface of the melted Mg alloy and about 660 to 690° C. near the lower region of feeder 23. Heating elements referenced and prefixed by the numeral 70 are preferably resistance heating elements.

[0035] In the barrel 30, the temperature is preferably maintained at about 620 to 680° C., preferably about 660 to 670° C. for the AZ91 Mg alloy. While only three heating elements 70 a-70 c are illustrated adjacent to the barrel in FIG. 2, there may be four or more heaters which heat the barrel 30.

[0036] In the injection chamber 50 and nozzle 57, the temperature is preferably maintained at about 620 to 700+ C., preferably about 660 to 690° C. for the AZ91 Mg alloy. While only three heating elements 70 h, 70 i and 70 j are illustrated in FIG. 2, there may be more than three heating elements which heat the injection chamber and nozzle. For example, there may be four heating elements which heat the nozzle and two heating elements which heat the portion of the injection chamber 50 in front of the seal 41. Preferably, the temperature in the nozzle is 10-30 degrees higher than in the injection chamber, and the nozzle tip is maintained at the highest temperature.

[0037] The temperature near heating elements 70 g and 70 f behind the seal 41 is preferably maintained at below 610° C., such as at about 600 to 570° C. for the AZ91 Mg alloy. The lower temperature behind the seal 41 helps prevent the metal from flowing past the seal 41. It should be noted that the liquid metal is prevented by the seal 41 from entering the portion of the injection chamber 50 adjacent to heating elements 70 f and 70 g, even when the plunger 45 is in a fully retracted position.

[0038] If desired, one or two additional heaters may be placed adjacent to the port 37. The port 37 may be maintained at about 620 to 670° C., preferably about 640 to 660° C.

[0039] The temperatures described above are sufficiently high to maintain the melted metal entirely in the liquid state from the time it exits the feeder 23 into the barrel 30 to the time the melted metal is injected into the mold 13 from the injection chamber 50. The temperatures may be varied depending on the type of metal part being molded.

[0040] Using the preceding temperatures at these locations permits molding of the AZ91 Mg alloy in the liquid state. Molded metallic parts having extremely smooth surfaces and minimal porosity can be produced, which allows them to be painted directly without any further machining. The molded metal parts also have extremely accurate dimensions and consistency, and can be produced with thicknesses of less than 1 mm when the part roughly has the dimensions of a DIN size A4 sheet of paper (21.0 cm by 29.7 cm). Preferably, the range of thickness of molded parts produced according to the preferred embodiment of the invention is between 0.5 and 1 mm for parts that have roughly the dimensions of a DIN size A4 sheet of paper. With known die casting and semi-solid methods, thicknesses no less than about 1.3 mm can be obtained for parts that have roughly the dimensions of a DIN size A4 sheet of paper.

[0041] As discussed above, use of the system and method of the preferred embodiment of the present invention, results in reduction or elimination of drooling. Another benefit is illustrated in FIGS. 4a and 4 b. FIG. 4b illustrates the pressure profile of an injection step using prior art injection molding with plug 91 formation as shown in FIG. 1a. Initially, there is a rapid increase in pressure due to the plug 91 being located in the nozzle exit opening 92. When the pressure builds up high enough to dislodge the plug, the plug bursts out of the nozzle into the sprue cavity. Then, there is a rapid decrease in pressure as metal shoots into the mold. This is followed by a gradual increase in pressure until the mold cavity is filled. Although this method works well for simple parts, the wide variation in pressure as a function of time has an adverse effect on the quality of more complicated parts, especially those with thin sections.

[0042]FIG. 4a illustrates the pressure profile of an injection step according to the above described preferred embodiment of the invention. Initially, there is a rapid increase in pressure until point 102, where the sprue cavity 14 is filled. Then the pressure rises rapidly again until point 104, where the mold cavity 15 fills. After the mold cavity 15 fills, there is one final pressure surge at point 106. The pressure then settles to a final value when the advancement of plunger 45 is stopped. The displacement of the plunger 45 is illustrated as the dashed line in FIG. 4a. Unlike the prior art, the pressure in the present embodiment of the invention does not decrease while the plunger is being advanced. The small drop in pressure after the pressure surge 106 occurs as or after the plunger 45 advancement has stopped (point 108).

[0043] The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The drawings and description were chosen in order to explain the principles of the invention and its practical application. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. 

What is claimed is:
 1. A method of injection molding a metal part comprising: (A) separating an injection nozzle of an injection chamber from contacting a mold surface; (B) retracting a plunger in the injection chamber to create a suction in the injection chamber; (C) closing an inlet to the injection chamber to seal the injection chamber; (D) maintaining melted metal in the injection chamber with a pressure difference and surface tension without substantial drooling from the injection nozzle; (E) placing the injection nozzle in contact with the mold surface; and (F) advancing the plunger in the injection chamber to inject a metal into the mold.
 2. The method of claim 1, wherein the metal is maintained substantially without drooling by a pressure difference between outside atmosphere and the injection chamber and surface tension in step (D) by a size of an opening of the injection nozzle being sufficiently small to substantially prevent the metal from drooling from the injection nozzle.
 3. The method of claim 2, wherein the plunger retraction in step (B) is started before or while the injection nozzle is separated from the mold in step (A).
 4. The method of claim 3, wherein the inlet to the injection chamber is closed in step (C) after the step of retracting the plunger in step (B).
 5. The method of claim 4, wherein the metal in step (D) comprises a liquid metal.
 6. The method of claim 5, wherein a portion of the liquid metal in a tip of the injection nozzle solidifies after injection; and the solidified metal remelts when the nozzle is separated from the mold.
 7. The method of claim 6, wherein an injection pressure in the injection chamber does not decrease during the step of advancing the plunger in step (F).
 8. The method of claim 7, wherein step (E) precedes step (F).
 9. The method of claim 1, further comprising repeating steps (A) through (F) a plurality of times.
 10. The method of claim 2, wherein the size of the opening of the injection nozzle is 15 mm or less.
 11. The method of claim 1, further comprising: (G) providing liquid metal into a temperature controlled barrel; and (H) providing the liquid metal from the barrel into the injection chamber through the inlet during step (B).
 12. The method of claim 11, further comprising: (I) stirring the liquid metal in the barrel by rotating a ram in the barrel; and (J) advancing the ram in the barrel to close the inlet to the injection chamber with a tip of the ram in step (C).
 13. The method of claim 1, wherein the suction in step (B) maintains the melted metal in the injection chamber without substantial drooling from the injection nozzle.
 14. The method of claim 13, wherein the suction in step (B) draws in the melted metal from a temperature controlled barrel through the inlet.
 15. The method of claim 1, further comprising maintaining a temperature of the injection nozzle of the injection chamber above a liquidus temperature of the melted metal such that no plug forms in the nozzle after step (A).
 16. The method of claim 12, wherein the plunger in the injection chamber retracts to create suction in step (B) before the ram advances in the barrel in step (J).
 17. The method of claim 16, wherein the barrel is located above the injection chamber to allow gravity to assist passage of the melted metal from the barrel into the injection chamber.
 18. The method of claim 5, wherein the metal comprises a magnesium alloy.
 19. A metal or metal alloy article made by the method of claim
 1. 20. The method of claim 10, wherein a diameter of the opening is 10 to 13 mm.
 21. A method of injecting melted metal into a mold comprising: introducing the melted metal into a barrel; allowing at least a first portion of the melted metal to pass through said barrel into an injection chamber; and injecting the melted metal from the injection chamber into the mold, wherein during the step of injecting, a pressure in the injection chamber does not decrease.
 22. The method of claim 21, further comprising advancing a ram in the barrel to seal an outlet port between the barrel and the injection chamber with a portion of the ram.
 23. The method of claim 22, wherein the advanced ram prevents the melted metal and gases from flowing between the barrel and the injection chambers during the step of injecting.
 24. The method of claim 21, further comprising maintaining a temperature of an injection nozzle of the injection chamber above a liquidus temperature of the melted metal.
 25. The method of claim 21, wherein said allowing step comprises creating a suction in the injection chamber to draw the portion of the melted metal from the barrel into the injection chamber.
 26. The method of claim 25, wherein a plunger in the injection chamber retracts to create suction that draws the melted metal.
 27. The method of claim 21, wherein the barrel is located above the injection chamber to allow gravity to assist passage of the melted metal from the barrel into the injection chamber.
 28. The method of claim 21, wherein the melted metal is in a liquid state.
 29. The method of claim 28, wherein the metal comprises a magnesium alloy.
 30. A metal article made by the method of claim
 21. 31. The method of claim 21, further comprising the steps of: (A) separating an injection nozzle of the injection chamber from contacting the mold surface; (B) retracting a plunger in the injection chamber to create a suction in the injection chamber; (C) closing an inlet to the injection chamber to seal the injection chamber; (D) maintaining melted metal in the injection chamber with a pressure difference and surface tension without substantial drooling from the injection nozzle; (E) placing the injection nozzle in contact with the mold surface; (F) advancing the plunger in the injection chamber to inject the melted metal into the mold.
 32. The method of claim 31, wherein the metal is maintained substantially without drooling by a pressure difference between outside atmosphere and the injection chamber and surface tension in step (D) by a size of an opening of the injection nozzle being sufficiently small to substantially prevent the melted metal from drooling from the injection nozzle.
 33. The method of claim 32, wherein the plunger retraction in step (B) is started before or while the injection nozzle is separated from the mold in step (A).
 34. The method of claim 33, wherein the inlet to the injection chamber is closed in step (C) after the step of retracting the plunger in step (B).
 35. The method of claim 34, wherein the melted metal in step (D) comprises a liquid metal.
 36. The method of claim 35, wherein a portion of the liquid metal in a tip of the injection nozzle solidifies after injection; and the solidified metal remelts when the nozzle is separated from the mold.
 37. The method of claim 36, wherein an injection pressure in the injection chamber does not decrease during the step of advancing the plunger in step (F).
 38. The method of claim 37, wherein step (E) precedes step (F).
 39. The method of claim 31, further comprising repeating steps (A) through (F) a plurality of times.
 40. The method of claim 32, wherein the size of the opening of the injection nozzle is 15 mm or less.
 41. The method of claim 31, further comprising: (G) providing liquid metal into a temperature controlled barrel; and (H) providing the liquid metal from the barrel into the injection chamber through the inlet during step (B).
 42. The method of claim 41, further comprising: (I) stirring the liquid metal in the barrel by rotating a ram in the barrel; and (J) advancing the ram in the barrel to close the inlet to the injection chamber with a tip of the ram in step (C).
 43. The method of claim 31, wherein the suction in step (B) maintains the melted metal in the injection chamber without substantial drooling from the injection nozzle.
 44. The method of claim 43, wherein the suction in step (B) draws in the melted metal from a temperature controlled barrel through the inlet.
 45. An injection molding apparatus comprising: an injection chamber; a plunger in the injection chamber; and an injection nozzle in fluid communication with the injection chamber having an opening sufficiently small to substantially prevent melted metal from drooling from the injection nozzle by a pressure difference between outside atmosphere and the injection chamber and surface tension.
 46. The apparatus of claim 45, wherein a diameter of the opening is 15 mm or less.
 47. The apparatus of claim 45, further comprising: a temperature controlled barrel; a ram in the barrel; and an inlet between the barrel and the injection chamber.
 48. The apparatus of claim 47, wherein the ram has a shape that is capable of blocking the inlet port to prevent a flow of the melted metal between the barrel and the injection chamber.
 49. The apparatus of claim 48, wherein a tip of the ram is shaped such that it seals the inlet port when the ram is in a fully advanced state.
 50. The apparatus of claim 49, wherein: the barrel is located above the injection chamber; and the plunger retracts to create suction that assists in drawing into the injection chamber at least a portion of the melted metal from the barrel through the inlet port.
 51. The apparatus of claim 50, wherein the plunger is advanced at a rate at which pressure in the injection chamber does not decrease.
 52. An injection molding apparatus, comprising: an injection chamber containing an injection nozzle; a first means for separating the injection nozzle from contacting a mold surface and for placing the injection nozzle in contact with the mold surface; a second means for injecting a melted metal from the injection chamber into the mold and for creating a suction in the injection chamber such that the melted metal is maintained in the injection chamber without substantially drooling from the injection nozzle between injection steps due to a pressure difference between an outside atmosphere and the injection chamber; a third opening means in the injection nozzle for maintaining the melted metal in the injection chamber without substantially drooling from the injection nozzle between injection steps due to surface tension; and a fourth means for closing an inlet to the injection chamber to seal the injection chamber.
 53. The apparatus of claim 52, wherein the third means comprises an opening in the injection nozzle that is sufficiently small to substantially prevent the melted metal from drooling from the injection nozzle due to surface tension.
 54. The apparatus of claim 53, wherein the second means retracts at a same time or before the injection nozzle is separated from the mold.
 55. The apparatus of claim 54, wherein the inlet to the injection chamber is closed when the second means retracts.
 56. The apparatus of claim 55, wherein the melted metal comprises a liquid metal.
 57. The apparatus of claim 56, wherein a portion of liquid metal in a tip of the injection nozzle solidifies after injection; and the solidified metal remelts when the nozzle is separated from the mold.
 58. The apparatus of claim 57, wherein an injection pressure in the injection chamber does not decrease when the second means injects the melted metal from the injection chamber into the mold.
 59. The apparatus of claim 53, wherein the size of the opening of the injection nozzle is 15 mm or less.
 60. The apparatus of claim 52, further comprising: a fifth means for providing liquid metal into a temperature controlled barrel; and a sixth means for providing the liquid metal from the barrel into the injection chamber.
 61. The apparatus of claim 60, wherein the suction draws in the melted metal from a temperature controlled barrel through the sixth means. 